If the underarms of one’s shirt (not under vest), get very sweaty during the 9 days – is one allowed to wipe the shirt with a baby wipe in order to remove the odor.
It is permitted to wipe the shirt in order to remove the odor.
This is similar to getting rid of a stain on one’s clothes, which is likewise permitted. Washing the clothing with water while one is wearing it cannot be compared to a full ‘laundry’ (Gesher Ha-Chaim 21:10-11), and even soap can be used if required. There is clearly no great ‘joy’ in the act of removing a stain. This ruling is also quoted from Rav Elyashiv shlita (as mentioned by Rabbi Shmuel Baruch Genut).
In addition, some authorities compare individual stains to children’s clothes, because of the frequency with which clothing gets stained (this comparison is made concerning Chol Ha-Moed, but can also be applied here).
Moreover, Chazal write that kavod ha-brios is so great, that it defers prohibitions, and this is also sufficient reason to permit wiping the shirt to remove the odor.